The present invention relates to an adaptive method and apparatus for n-phasic truncated-exponential defibrillation that optimizes the shock delivery technique based on a patient's system impedance.
In conventional defibrillators, defibrillation waveforms are delivered in one of a fixed-tilt or fixed-duration format. As is well known in the art, an n-phasic waveform comprises consecutive pulses of opposite polarity. Characteristics of an n-phasic defibrillation waveform include the rate or angle of decay of an individual pulse (often referred to as the "tilt," where tilt=((V.sub.i -V.sub.f)(V.sub.i).times.100%) and the duration of each individual pulse. In devices using the fixed-duration format, the duration of each pulse is fixed but the tilt of each pulse varies with patient system impedance. See, for example, U.S. Pat. Nos. 4,800,883 to Winstrom and 4,821,723 to Baker, Jr. et al. Studies have shown that the defibrillation energy increases as the trailing edge voltage becomes lower, like that associated with low patient system impedances. A drawback of the fixed duration technique is that for patients with low impedances, shock durations must be manually programmed to shorter values to prevent the tilts from becoming too large, creating low trailing voltages.
In devices which use the fixed-tilt format, the individual pulses of the n-phasic waveform have durations that vary with patient system impedance. See, for example, U.S. Pat. No. 4,998,531, granted Mar. 12, 1991, to Bocchi et al. This type of device avoids low trailing edge voltage by automatically varying the duration. However, it is sometimes unnecessary to continue capacitor discharge beyond the 15-20 msec duration. While the longer durations for fixed-tilt waveforms at high impedances are not detrimental, they do have the disadvantage of delivering unneeded extra energy to the patient.
While some defibrillation systems utilize fixed-tilt or fixed-duration shock delivery techniques, there is no system heretofore known which combines the principles of both of these techniques. Fixed-duration devices require manual programming to assure correct waveforms at low impedances. Fixed-tilt devices do not incorporate a time limit for switching to an alternative shock delivery method. Combining the best features of both techniques into a device with adaptive n-phasic waveforms yields a therapy option that is automatically optimized for each patient.